Monitoring led wavelength shift in photoplethysmography

Abstract

The present invention is directed to a method and system for the determination of a spectral characteristic of light signals emitted by light signal emitters of a photoplethysmographic probe to achieve improved accuracy in photoplethysmographic blood analyte level determinations. In one embodiment, a system for use in photoplethysmography includes a plurality of light signal emitters ( 20 A-D) within a photoplethysmographic probe ( 12 ) and a voltage sensor ( 60 ) and data processor ( 70 ) within a photoplethysmographic monitor unit ( 14 ) to which the probe ( 12 ) is connectable. The voltage sensor ( 60 ) is operable to sense a voltage drop across each light signal emitter ( 20 A-D) as each emits a corresponding light signal ( 22 A-D) for transmission through a tissue under test. The data processor ( 70 ) is operable to establish at least one spectral characteristic, such as the center wavelength, of each emitted light signal ( 22 A-D) based on the sensed voltage drops.

What is claimed is:  
     
         1 . A method for use in photoplethysmographic measurement of blood analyte levels comprising the steps of: 
 measuring a voltage drop across a light signal emitter as the light signal emitter emits a light signal for use in determining a blood analyte level;    establishing a spectral characteristic of the emitted light signal based on the measured voltage drop across the light signal emitter; and    using the established spectral characteristic of the emitted light signal in determining a blood analyte level.    
     
     
         2 . The method of  claim 1  wherein the established spectral characteristic comprises a center wavelength of the emitted light signal.  
     
     
         3 . The method of  claim 1  wherein the light signal emitter comprises an LED.  
     
     
         4 . The method of  claim 1  further comprising the steps of: 
 obtaining data correlating the measured voltage drop across the light signal emitter with the spectral characteristic of the light signal emitted by the light signal emitter; and  
 storing the data wherein the data is usable to establish the spectral characteristic of the emitted light signal in said step of establishing a spectral characteristic of the emitted light signal.  
 
     
     
         5 . The method of  claim 4  wherein the data comprises a plurality of pairs of data points, each pair of data points including a first value representing a voltage drop across the light signal emitter and a second value representing the spectral characteristic of a corresponding light signal emitted by the light signal emitter.  
     
     
         6 . The method of  claim 4  wherein the data comprises a slope and an intercept point of a plot of a measured voltage drop across the light signal emitter versus the spectral characteristic of a corresponding light signal emitted by the light signal emitter.  
     
     
         7 . The method of  claim 4  wherein said step of obtaining comprises the steps of: 
 operating the light signal emitter a plurality of times to emit a plurality of light signals, wherein the light signal emitter is operated under varying operating conditions to vary the spectral characteristic of the plurality of light signals output by the light signal emitter;  
 detecting the voltage drop across the light signal emitter each time the light signal emitter is operated; and  
 analyzing the spectrum of each light signal emitted by the light signal emitter each time the light signal emitter is operated to obtain the spectral characteristic of each light signal emitted by the light signal emitter.  
 
     
     
         8 . The method of  claim 7  wherein the varying operating conditions include various temperatures.  
     
     
         9 . The method of  claim 4  wherein the light signal emitter is included in a photoplethysmographic probe including a data storage device and, in said step of storing, the data is stored on the data storage device of the probe.  
     
     
         10 . The method of  claim 9  wherein the data storage device comprises an EPROM.  
     
     
         11 . A method of providing a spectral characteristic of a light signal emittable from a light signal emitter of a photoplethysmographic probe to a photoplethysmographic measurement unit wherein the spectral characteristic is usable in the determination of a blood analyte level by the photoplethysmographic measurement unit, said method comprising the steps of: 
 obtaining data correlating a voltage drop across the light signal emitter with the spectral characteristic of the light signal;    storing the data wherein the data is usable to establish the spectral characteristic of the emitted light signal when the photoplethysmographic probe is used in determining a blood analyte level;    measuring the voltage drop across the light signal emitter as the light signal emitter emits the light signal; and    using the stored data to establish the spectral characteristic of the emitted light signal based on the measured voltage drop across the light signal emitter.    
     
     
         12 . The method of  claim 11  wherein the spectral characteristic comprises a center wavelength of the emitted light signal.  
     
     
         13 . The method of  claim 11  wherein the light signal emitter comprises an LED.  
     
     
         14 . The method of  claim 11  wherein the data comprises a plurality of pairs of data points, each pair of data points including a first value representing a voltage drop across the light signal emitter and a second value representing the spectral characteristic of a corresponding light signal emitted by the light signal emitter.  
     
     
         15 . The method of  claim 11  wherein the data comprises a slope and an intercept point of a plot of a measured voltage drop across the light signal emitter versus the spectral characteristic of a corresponding light signal emitted by the light signal emitter.  
     
     
         16 . The method of  claim 11  wherein said step of obtaining comprises the steps of: 
 operating the light signal emitter a plurality of times to emit a plurality of light signals, wherein the light signal emitter is operated under varying operating conditions to vary the spectral characteristics of the plurality of light signals output by the light signal emitter;  
 detecting the voltage drop across the light signal emitter each time the light signal emitter is operated; and  
 analyzing the spectrum of the light signal emitted by the light signal emitter each time the light signal emitter is operated to obtain the spectral characteristic of the light signal emitted by the light signal emitter.  
 
     
     
         17 . The method of  claim 16  wherein the varying operating conditions include various temperatures.  
     
     
         18 . The method of  claim 11  wherein, in said step of storing, the data is stored on the data storage device of the probe.  
     
     
         19 . The method of  claim 18  wherein the data storage device comprises an EPROM.  
     
     
         20 . A system for use in photoplethysmographic measurement of at least one blood analyte level in a tissue under test comprising: 
 a plurality of light signal emitters operable to emit a corresponding plurality of light signals for transmission through the tissue under test;    a voltage sensor operable to sense a voltage drop across each said light signal emitter as each said light signal emitter emits a corresponding light signal; and    a data processor operable to establish at least one spectral characteristic of each said emitted light signal based on the sensed voltage drop across said light signal emitter corresponding therewith, wherein said at least one spectral characteristic is usable in determining at least one blood analyte level.    
     
     
         21 . The system of  claim 20  wherein said at least one spectral characteristic comprises a center wavelength of each said emitted light signal.  
     
     
         22 . The system of  claim 20  wherein each said light signal emitter comprises an LED.  
     
     
         23 . The system of  claim 20  wherein said light signal emitters are disposed within a photoplethysmographic probe and said voltage sensor is disposed in a photoplethysmographic monitor unit to which said photoplethysmographic probe is connectable.  
     
     
         24 . The system of  claim 20  further comprising: 
 a data storage device for storing data correlating the sensed voltage drop across each said light signal emitter with said at least one spectral characteristic of each said emitted light signal, wherein said data is accessible to said data processor for use in establishing said at least one spectral characteristic of each said emitted light signal based on the sensed voltage drop across said light signal emitter corresponding therewith.  
 
     
     
         25 . The system of  claim 24  wherein said data comprises: 
 a plurality of pairs of data points, each said pair of data points corresponding with one of said light signal emitters and including a first value representing a voltage drop across said corresponding light signal emitter and a second value representing said at least one spectral characteristic of a corresponding light signal emitted by said corresponding light signal emitter.  
 
     
     
         26 . The system of  claim 24  wherein said data comprises: 
 a plurality of slope values and intercept points, each said slope value and intercept point being associated with a plot of the voltage drop across a corresponding one of said light signal emitters versus said at least one spectral characteristic of the light signal emitted by said corresponding light emitter.  
 
     
     
         27 . The system of  claim 24  wherein said data storage device is disposed within a photoplethysmographic probe and said data processor is disposed within a photoplethysmographic monitor unit to which said photoplethysmographic probe is connectable.  
     
     
         28 . The system of  claim 24  wherein said data storage device comprises an EPROM.  
     
     
         29 . A photoplethysmographic probe connectable with a photoplethysmographic monitor unit for use in photoplethysmographic measurement of at least one blood analyte level in a tissue under test comprising: 
 a plurality of light signal emitters operable to emit a corresponding plurality of light signals for transmission through the tissue under test; and    a data storage device for storing data correlating a sensed voltage drop across each said light signal emitter when operated to emit a light signal with at least one spectral characteristic of each said emitted light signal, wherein said data is accessible to said monitor unit for use in establishing said at least one spectral characteristic of each said emitted light signal based on the sensed voltage drop across each said light signal emitter.    
     
     
         30 . The photoplethysmographic probe of  claim 29  wherein said at least one spectral characteristic comprises a center wavelength of each said emitted light signal.  
     
     
         31 . The photoplethysmographic probe of  claim 29  wherein each said light signal emitter comprises an LED.  
     
     
         32 . The photoplethysmographic probe of  claim 29  wherein said data comprises: 
 a plurality of pairs of data points, each said pair of data points corresponding with one of said light signal emitters and including a first value representing a voltage drop across said corresponding light signal emitter and a second value representing said at least one spectral characteristic of a corresponding light signal emitted by said corresponding light signal emitter.  
 
     
     
         33 . The photoplethysmographic probe of  claim 29  wherein said data comprises: 
 a plurality of pairs of slope values and intercept points, each said slope value and intercept point of a pair being associated with a plot of the voltage drop across a corresponding one of said light signal emitters versus said at least one spectral characteristic of the light signal emitted by said corresponding light emitter.  
 
     
     
         34 . The photoplethysmographic probe of  claim 29  wherein said data storage device comprises and EPROM.